This invention relates to an apparatus and method for separating entrained particulate matter from a conveying fluid. The specific disclosure of this application is that of a filter for removing cotton dust and fiber from air. However, the invention as claimed is not limited to the filtration of any particular type of particulate matter, nor is the invention limited to the filtration of air--the filtration of other fluids, such as other gasses or liquids being contained within the scope of this invention.
The filtration of fluids, especially air, has become an increasingly important aspect of Government regulation of air quality standards. This is especially true of the environment within textile mills, where the presence of cotton dust and fibers allegedly constitutes an occupational health hazard.
It has been previously recognized that filtering efficiency is increased as a buildup of filtered material (referred to as a "mat") occurs on the upstream side of a screen-type filter. This increase in efficiency is a result of the air being subjected to filtration through a finer filter medium and over a much greater linear distance. Throughout this distance, the mat projects innumerable obstacles into the air stream which deflect and trap entrained particulate matter. Prior art patents have sought to take advantage of this phenomenon.
Representative is the Neitzel U.S. Pat. No. 3,525,198. Neitzel discloses the insertion of a primary fly lint separator into a pneumatic duct which connects a carding room in a textile mill with conventional lint filtering means. The lint separator comprises a rotatable screen drum through which air is passed, with the filtered lint collecting on the upstream surface of the drum. The Neitzel patent states that the continuous rotation of screen drums has not proven practical because a lap having sufficient thickness for efficient doffing cannot build upon the drum surface, notwithstanding a very slow rate of rotation. Neitzel therefore provides a rotatable screen drum which is intermittently rotated through a 180.degree. arc after a substantial mat of fibers has collected on the outside of the screen while stationary. While the Neitzel patent does not explicitly recognize the enhanced filtering efficiency which results from the buildup of lint on the exposed surface of the drum, it is apparent from the disclosure that such an improvement takes place until the drum is rotated, whereupon an immediate decrease in filtering efficiency results as a result of the passage of air through only the screen drum itself. Moreover, the Neitzel patent recognizes that an indefinite accumulation of fibers on the surface of a screen drum results in a gradual decrease of air velocity as the filter becomes clogged.
The Broadbent U.S. Pat. No. 3,628,313 explicitly recognizes the enhanced filtering effect achieved when fibers are collected from an air stream and then used to augment the filtering process. However, the Broadbent patent also recognizes, as is recognized in Neitzel, that there is a increase in air pressure incident to the buildup of a thick mat of fibers on the drum. Therefore, means for sensing this increase in air pressure is provided which momentarily rotates the drum to remove the mat from a predetermined portion of the drum, and exposes the previously matted portion of the filter to the flow of air in order to increase the rate of air flow through the filter. As in Neitzel, there is a degradation of filtering efficiency when the previously matted portion of the filter is exposed to the stream of air. The Broadbent apparatus attempts to diminish this effect by passing the air around a substantial portion of the periphery of the drum at an oblique angle prior to the air reaching the exposed portion of the filter. While it is stated that this procedure avoids the necessity of filtering the air through a secondary filtering means, nevertheless the two broadly recognized problems in the prior art, i.e., the gradual decrease in air velocity as the mat builds up on the filter drum, and the uneven quality of the filtered air resulting from the periodic movement of the drum to expose the uncovered drum to the stream of air, remain largely unsolved.
In apparent recognition of these problems, the Ferri et al U.S. Pat. No. 4,090,857, provides a stationary cylindrical filter box having an air inlet extending in a tangent with respect to the filter in order to induce the air entering the filter to spin. According to Ferri, this spinning motion simultaneously forces air through the filtering medium and constantly moves the trapped fibers along the filter surface to a settling chamber, thereby resulting in a constant pressure drop through the filter. Thus, Ferri avoids the problems inherent in the Neitzel and Broadbent patents by preventing the coating of the filter with trapped fibers. While achieving the goal of maintaining a constant pressure within the system, the advantages inherent in using a thick mat of fibers to enhance the filtration process are completely lost.